Move Salt-internals to Salt's reclass adapters

The Salt reclass_adapter plugins now hide all the internals and provide
reclass only with the information it needs, without having to make
assumptions about Salt internals.

Signed-off-by: martin f. krafft <madduck@madduck.net>
Add README for Salt integration/adapter
README update about contributing and pull requests

Signed-off-by: martin f. krafft <madduck@madduck.net>
Add variable interpolation TODO

Signed-off-by: martin f. krafft <madduck@madduck.net>